The present invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material, utilizing a coloring reaction between a leuco dye and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto, which thermosensitive recording material has excellent coloring sensitivity and is capable of yielding images with high density and high development uniformity.
A conventional thermosensitive recording material comprises a support material, such as a sheet of paper or a film, and a thermosensitive coloring layer formed thereon, which comprises a coloring system consisting of a colorless or light-colored coloring dye such as a leuco dye and a color developer capable of inducing color formation in the coloring dye upon application of heat thereto. In addition to the above coloring system, the thermosensitive coloring layer may further comprise a binder agent, a filler, a thermal sensitivity improvement agent, a lubricant and other auxiliary agents. Examples of such a thermosensitive recording material are disclosed, for example, in Japanese Pat. Publications No. 43-4160 and No. 45-14039.
Such thermosensitive recording material has the advantages over other conventional recording materials that recording can be performed in a short time by use of a comparatively simple device without the need for complicated processes such as developing and fixing, almost no noise is generated and no environmental pollution problems occur during operation, and the cost is low. Because of these advantages, the thermosensitive recording material has wide-scale utilization, not only as the recording material for copying books and documents, but also as the recording material for use with computers, facsimile apparatus, telex, medical instruments, other apparatus for recording information, and measuring instruments.
In accordance with the recent demand for high speed recording with high recording density, not only the development of a high speed recording apparatus, but also the development of a recording material that can be used with such a high speed recording apparatus is desired. For instance, there is a demand for a thermosensitive recording material that can yield clear images high image density with application of a small amount of thermal energy and has excellent head-matching properties, that is, the properties of not sticking to a thermal head for thermal image formation and not forming any dust from the recording material in the course of thermosensitive recording by use of a thermal head which comes into contact with the surface of the thermosensitive recording material.
A method by which the above demand could be met is to increase the thermosensitivity of the recording material by addition of a thermofusible material which melts at a temperature lower than the melting points of the employed leuco dye and color developer and is capable of dissolving therein the leuco dye and color developer when melted. A variety of such thermofusible materials have been proposed, for example, nitrogen-containing compounds such as acetamide, stearamide, m-nitroaniline, and phthalic acid dinitrile in Japanese Laid-Open Pat. application No. 49-34842; acetoacetic anilide in Japanese Laid-Open Pat. application No. 52-106746; and alkylated biphenyl alkanes in Japanese Laid-Open Pat. application No. 53-39139.
However, in accordance with the recent development of a high speed thermal facsimile apparatus, it has become a common practice to energize and drive a thermal head at high speed. Under such circumstances, there is a problem that the background of a thermosensitive recording sheet is also colored in the course of high speed thermosensitive recording by use of a thermal head due to the build-up of thermal energy in the thermal head and therearound. In other words, the dynamic thermosensitivity of such conventional thermosensitive recording material is still poor. Therefore it is one of the most important subjects to be cleared to enhance the dynamic coloring thermosensitivity in the conventional thermosensitive recording materials, without decreasing the coloring initiation temperature thereof. In the conventional thermosensitive recording materials, the dynamic coloring thermosensitivity cannot be increased unless a large amount of the above-mentioned thermofusible compounds is added, although the static coloring thermosensitivity can be increased to some extent by the addition of the thermofusible compounds. However, when a large amount of the thermofusible materials is employed, the sticking and dust-adhesion problems occur in the course of thermal recording by a thermal head. Further, when the melting points of the thermofusible compounds are extremely low, the thermosensitivity can be in fact increased by the addition of such thermofusible compounds, but the preservability of the thermosensitive recording material is significantly reduced so that the fogging of the background of the recording material may occur during the storage thereof.
For the purpose of improving the dynamic coloring thermosensitivity of the recording material, there have been proposed a method of improving the smoothness of the thermosensitive coloring layer, and a method of increasing the concentration of the coloring components by decreasing the relative amounts of components which do not contribute to the coloring reaction of the thermosensitive recording material, such as fillers and binder agents.
The method of improving the smoothness of the thermosensitive coloring layer is usually carried out without difficulty by use of a super calender. However, this method has the shortcomings that the background of the thermosensitive coloring layer is colored during the calendering process and the surface of the thermosensitive coloring layer becomes so glossy that the appearance of the recording material is impaired.
To the thermosensitive coloring layer, a filler such as calcium carbonate, clay and urea-formaldehyde resin may be added to maintain the whiteness of the background of the coloring layer and to prevent the sticking and dust adhesion problems during the thermosensitive recording using a thermal head. Further a water-soluble binder agent may be added to the thermosensitive coloring layer to firmly bind the coloring components and other additives of the thermosensitive coloring layer to a support. However, when the amount of such a filler and a water-soluble binder agent is reduced, the desired properties for the thermosensitive recording material cannot be obtained.
Japanese Laid-Open Pat. application No. 59-5093 discloses a thermosensitive recording material comprising a support, a highly heating insulating foamed layer which is formed on the support by coating an expandable plastic filler and expanding the filler with application of heat thereto, and a thermosensitive coloring layer formed on the foamed layer. This structure is intended to obtain a highly thermosensitive recording material, in particular, by use of the above heat insulating foamed layer. However, the surface of the heat insulating foamed layer is so unsmooth that uniform images can be hardly obtained.
Further Japanese Laid-Open Pat. application No. 59-225987 discloses a thermosensitive recording material comprising a support, a foamed layer which is formed by coating an expandable plastic filler on the support and expanding the filler with application of heat thereto, an undercoat layer comprising a filler and a binder agent, and a thermosensitive coloring layer comprising a leuco dye and a color developer, which layers are successively overlaid on the support in this order. In this prior art reference, it is mentioned that it is preferable that the amount of the binder agent contained in the undercoat layer be about 5 to 50 wt. %. By use of this undercoat layer, the surface of the foamed layer can be made smooth to some extent, but the smoothness obtained by this undercoat layer is still insufficient for obtaining enough dynamic coloring thermosensitivity and coloring uniformity for use in practice.